Buffer very large delta streams to reduce explosion of CPU work

Buffer very large delta streams to reduce explosion of CPU work

Large delta streams are unpacked incrementally, but because a delta
can seek to a random position in the base to perform a copy we may
need to inflate the base repeatedly just to complete one delta.
So work around it by copying the base to a temporary file, and then
we can read from that temporary file using random seeks instead.
Its far more efficient because we now only need to inflate the
base once.

This is still really ugly because we have to dump to a temporary
file, but at least the code can successfully process a large
file without throwing OutOfMemoryError.  If speed is an
issue, the user will need to increase the JVM heap and ensure
core.streamFileThreshold is set to a higher value, so we don't use
this code path as often.

Unfortunately we lose the "optimization" of skipping over portions
of a delta base that we don't actually need in the final result.
This is going to cause us to inflate and write to disk useless
regions that were deleted and do not appear in the final result.
We could later improve on our code by trying to flatten delta
instruction streams before we touch the bottom base object, and
then only store the portions of the base we really need for the
final result and that appear out-of-order.  Since that is some
pretty complex code I'm punting on it for now and just doing this
simple whole-object buffering.

Because the process umask might be permitting other users to read
files we create, we put the temporary buffers into $GIT_DIR/objects.
We can reasonably assume that if a reader can read our temporary
buffer file in that directory, they can also read the base pack
file we are pulling it from and therefore its not a security breach
to expose the inflated content in a file.  This requires a reader
to have write access to the repository, but only if the file is
really big.  I'd rather err on the side of caution here and refuse
to read a very big file into /tmp than to possibly expose a secured
content because the Java 5 JVM won't let us create a protected
temporary file that only the current user can access.

Change-Id: I66fb80b08cbcaf0f65f2db0462c546a495a160dd
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>